Generator action is employed in an electrical generator to produce an electrical output current from one or more stator windings. Conventionally, a generator comprises at least one stator winding carried on a stator core and a rotor shaft carrying a rotating field winding. Current is supplied to the rotating field winding from an exciter, as described further below, for creating a magnetic flux field that rotates within the stator core. One end of the rotor shaft is drivingly coupled to a steam or gas driven turbine for providing rotational energy to the rotor shaft and thus to the rotating field winding. The opposing end of the rotor shaft is coupled to the exciter. The rotating magnetic field induces current flow in the stator windings as the rotating field winding cuts across the stator windings. Single phase AC is supplied from a single stator winding. If the stator comprises three independent stator windings spaced at 120° intervals around the stator core, the stator output is three-phase alternating current. The exciter provides direct current (DC) to the rotating field winding of the generator. Like the main generator, the exciter employs generator action to develop the DC output. The exciter DC output is regulated to control the intensity of the magnetic field developed by the main generator field winding. Since the stator winding is responsive to this magnetic field, the main generator AC output current is thus controlled by the DC input to the rotating field winding.
In one embodiment, a so-called brushless exciter, the exciter comprises a rotating winding situated on the same turbine-driven shaft as the main generator, and a stationary field winding responsive to an externally-generated DC current. As the exciter winding rotates through the stationary magnetic field of the field winding, an AC current is induced in the former. The AC is converted to DC by a rectifier bridge mounted on the rotating shaft, and the resulting DC current is supplied to the generator field winding through conductors also mounted on the rotating shaft.
A brush and slip ring exciter 10 is illustrated in FIG. 1, comprising a rotating armature 12 carrying armature windings 13 and a stationary stator 14 carrying stator windings 15. DC power is supplied from a power supply 16 to the stator windings 15, for creating a stationary magnetic field, where reference character 18 identifies field lines of the stationary magnetic field. Generator action produces a current in the armature windings 13 as the stationary magnetic field lines cut the armature windings 13. The armature winding current is carried over conductors 22 to a commutator 24, comprising two commutator segments 24A and 24B. Brushes 26A and 26B in physical contact with the commutator segments 24A and 24B, respectively, carry the current over conductors 30 and 32 to a second set of brushes 34 and 36. The brush 34 carries the current on the conductor 30 to an input winding 40 of the generator rotor field winding 42 via a slip ring 44. The brush 36 carries the current on the-conductor 32 to an input winding 46 of the generator rotor field winding 42 via a slip ring 48. As described above, a turbine (not shown in FIG. 1) imparts rotation to the generator rotor field winding 42, thus inducing current in the stationary stator windings (not shown in FIG. 1) of the generator.
It is noted that AC current is produced in the armature windings 13 and is rectified to DC by the action of the commutator 24. Consider a shaded side 60 and an unshaded side 62 of the rotating armature 12 as indicated in FIG. 1. As the shaded side 60 cuts through the magnetic field created by the stator windings 15, current flows in a direction indicated by the arrowheads 64. That is, current flows from the unshaded side 62 toward the shaded side 60. At this point in the rotational cycle of the rotating armature 12, the brush 26B is in electrical communication with the shaded side 60 and the brush 26A is in electrical communication with the unshaded side 62. Thus current flows from the armature windings 13 to the brush 26B, through the conductor 32, to the brush 36 and the slip ring 48 to the input winding 42 of the generator rotor field winding 42. The return current flows from the input winding 40 through the slip ring 44 and the brush 34, through the conductor 30 to the brush 26A and the commutator segment 24A and back to the rotating armature 12.
As the rotating armature 12 rotates through 180° due to the rotational energy supplied by the turbine (not shown), current is induced to flow in the opposite direction through the armature windings 13. See FIG. 2. Current flows from the shaded side 60 to the unshaded side 62 of the rotating armature 12, to the commutator segment 24A, the brush 26A and follows the conductive path indicated by arrowheads 70. Note that the current flows in the same direction through the generator rotor field winding 42 in both the FIG. 1 and the FIG. 2 orientations. Thus the commutator 24 effects a rectification of the AC current flowing from the armature windings 13.
Brushes, such as the brushes 26A/B and the brushes 34/36 described above, are commonly used in dynamoelectric machines (i.e., motors and generators) in conjunction with a commutator or with slip rings as illustrated in FIGS. 1 and 2. The material of the brushes should provide good electrical conductivity and a long-life to avoid the need for frequent brush replacement. Also, the material should be relatively soft compared with the material of the rotating commutator against which the brushes are in physical contact. Use of a relatively soft material causes the replaceable brush to wear, rather than the commutator. Typically, the brushes are formed from carbon, graphite or an alloy of copper and carbon. In some applications a plurality of brushes are connected in an electrical parallel orientation to provide higher current capacity.
The brushes are held in place against the commutator by a brush holder mounted on the frame of the dynamoelectric machine and electrically insulated there from. Conventionally, the brush holder includes a spring associated with each brush for supplying a constant force to hold the brush against the commutator. However, since the bushes wear as the commutator rotates and eventually must be replaced, a brush holder providing efficient and rapid brush replacement is desired. Also, in certain dynamoelectric machines, brush adjustment and replacement can be performed only when the machine is not in operation. Thus a brush holder that allows rapid brush replacement limits machine downtime.
Accurate brush position relative to the rotating commutator is required for maximum commutation action and minimum brush wear. The brush must be set at a distance from the commutator that minimizes brush wear while providing good electrical contact between the brush and the commutator. The brush must not exert an undue force on the commutator, which would create excess friction and rapid brush wear. However, an insufficient contact force creates a gap between the brush face and the commutator. The gap introduces excess resistance into the brush circuit and arcing between the brush and the commutator, leading to premature brush wear.
It is also important to control the position of the brush axially along the commutator. For single ring commutators and slip rings, the brush should be positioned approximately at the center of the commutator or ring. For commutators having multiple adjacent rings, the brush should be positioned at the ring center to avoid contact with adjacent rings and the brushes associated therewith.